A bag contains six balls of different colours. Two balls are drawn in succession with replacement. The probability that both the balls are of the same colour is \( p \). Next four balls are drawn in succession with replacement and the probability that exactly three balls are of the same colour is \( q \). If \( p : q = m : n \), where \( m \) and \( n \) are coprime, then \( m + n \) is equal to ________.
